Generally, a front derailleur for a bicycle is provided with a movable member which has inner and outer guide plates disposed radially outwardly from the addendum circle of the front chain gear and in the vicinity of the entrance of a driving chain engageable with the chain gear. The movable member is supported through a pair of linkage members to a fixing member in relation of being movable axially of the front gear, so that the inner surface of the inner guide plate is brought into contact with one side surface of the chain carried by a smaller diameter chain gear and the chain disengages therefrom and is urged toward a larger diameter chain gear so as to be shifted thereto from the smaller diameter chain gear, while, the inner surface of outer guide plate is brought into contact with one side surface of the chain, so that the chain is urged toward the smaller diameter chain gear and disengages from the larger diameter chain gear, thereby being shifted therefrom to the smaller diameter chain gear.
Each of the guide plates is formed at the lower edge thereof in a circular arc and they are mounted to the bicycle frame such that the lower edge of outer guide plate is spaced in a proper interval from the larger diameter chain gear.
When the conventional two-stage speed change front derailleur is used for a three-stage speed change derailleur, especially when the chain is shifted from the smaller diameter chain gear to a middle diameter chain gear, the inner guide plate shifts too much radially outwardly from each tooth crest of the middle diameter chain gear to enlarge a distance between the chain urging portion at the inner guide plate and the chain engaging point at the middle diameter chain gear. As a result, the chain to be shifted deflects excessively to cause a larger movement of the inner guide plate, and in turn a longer operating stroke of the inner guide plate to that extent, thereby lowering the speed change efficiency.
As a countermeasure for the above, the inner guide plate, when shifted toward the middle diameter chain gear, is kept at the lower edge in a proper interval from each tooth crest of the same, thereby enabling a good speed change efficiency to be obtained. In this case, however, the lower edge of the inner guide plate is positioned radially inwardly from the addendum circle of the larger diameter chain gear.
At this time, the chain is pressed hard to the inner surface of the larger diameter chain gear, resulting in that the chain is jammed between the inner guide plate and the larger diameter chain gear, thereby occasionally making chain shifting impossible.